1. Field of the invention
This invention relates to mass spectrometer systems. The invention has particular, although not exclusive, relevance to mass spectrometer systems which include an ion source operating under high pressure conditions, for example an electrospray ionization source.
Electrospray ionization sources produce intact ions from a complex, high molecular weight analyte in solution, by spraying the analyte in solution from a needle to which a high voltage is applied, to produce highly-charged droplets, and causing desorption of the charged analyte species from the droplets. Thus the source operates under high pressure conditions, typically close to atmospheric conditions, the ions thus originating in a viscous flow region.
The efficient coupling of any high pressure ion source, that is where ions originate in a viscous flow region, to the mass analyser of a mass spectrometer operating under high vacuum conditions, requires some form of enrichment of the ion-to-neutral gas molecule ratio in the flow which enters the mass analyser. This enrichment may be produced mechanically by use of the mass differences between the ions and neutral gas molecules. Alternatively, the enrichment may be produced electrically, by use of charging forces which preferentially direct the ions through a series of orifices leading into the high vacuum system, these orifices being very small compared with the orifices through which excess neutral gas molecules are pumped away, i.e. the technique known as "differential pumping" between regions of different degrees of vacuum.
Generally, where ions to be analysed in a mass spectrometer system originate from a viscous flow region in which the ion energy is approximately zero because of the large number of thermalising collisions by the ions with the surrounding gas molecules, it is preferred to introduce the ions directly into a region where molecular flow prevails. In the molecular flow region the only ion collisions will be with the walls of the container, and thus the energy of the ions may be precisely defined, as is necessary for mass spectrometry analysis of the ions. This introduction may be achieved by means of plates in the molecular flow region, the plates being held at an appropriate potential.
2. Description of the prior art
Where an ion source operates at a pressure close to atmospheric pressure, it is known to connnect the atmospheric pressure region directly to a molecular flow region by use of a pumping means having a very large pumping speed in the molecular flow region, and additionally or alternatively connecting the viscous flow and molecular flow regions via a very small orifice. Large pumping speeds in the molecular flow region, however, necessitate the use of large, expensive cryopumps. Use of a small orifice between the viscous flow and molecular flow regions, on the other hand, reduces ion transmission into the mass analyser, and thus reduces the sensitivity of the mass spectrometer system.
Examples of mass spectrometer systems in which a small orifice separates the viscous flow and molecular flow regions are described in articles in "Electrospray Interface for Liquid Chromatographs and Mass Spectrometers" Anal. Chem. 1985, Volume 57, pages 675-679, and "Electrospray Interface for Liquid Chromatography/Mass Spectrometry", Rapid Communications in Mass Spectrometry Volume 4, No. 12, pages 519-526, 1990. In both cases the sensitivities of the systems are reduced by the necessity of using small orifices to separate the viscous flow and molecular flow regions.
A further article "Electrospray on a Magnetic Sector Instrument", Biological Mass Spectrometry, (Edited Burlingame and McCloskey), pages 147-157, describes a mass spectrometer system including a magnetic sector analyser and an ion spray source, in which lower sensitivity was encountered than with a quadrupole instrument operating with a similar source. This was attributed to the higher pressures used in the spectrometer system which may indicate the absence of a true molecular flow region.
An article in "Electrospray Ionization on a High-Performance Magnetic-sector Mass Spectrometer", Rapid Communications in Mass Spectrometry, Volume 4, No. 5, pages 147-150, 1990, describes a system in which the viscous and molecular flow regions are separated by an intermediate pressure region. The system described has low sensitivity, highly charged ions not being detected, this being attributable to a lack of a clearly defined transition from viscous to molecular flow, with the molecular flow region being too long.